Electromechanical device



March ,934. A. D. BLUMLE'IN El AL 1,952,357

ELECTROMECHANICAL DEVICE Original Filed Feb. 26', 1931 fave/1507a? A [EA Danae 52 (/ML [IA/4N0 #EEBIETZZJWAED/IOA MA/V Patented ar. 27, 1934 ELECTROMECHANICAL DEVICE Alan Dower Blumlein and Herbert Edward Holman, London, England, assignors to Columbia Graphophone Company, Limited, London,

England Britain May 7, 1930 16 Claims.

This invention relates to electro-acoustic devices employed for the interconversion of acoustic and electrical vibrations, such as microphones, loud-speakers and the like, and is especially directed to microphonic apparatus of this type intended for use with the sound recording apparatus described in our British Patent applications Nos. 7809/ and 14042/30.

The invention consists in an electro-acoustic device having a vibrating system consisting of a substantially rigid piston-like diaphragm connected at its outer edge to a flexible air sealing surround wherein means are provided for suppressing or controlling subsidiary resonances of the system which means do not contribute the bulk of the damping of the main resonance.

The invention also consists in an electroacoustic device having a vibrating system consisting of a substantially rigid piston-like diaphragm connected at its outer edge to a flexible air sealing surround wherein means are provided for applying shielding and/or damping to the surround only.

The invention further consists in an electroacoustic device as set forth above wherein the piston-like portion of the diaphragm is composed of light wood, such as balsa wood, covered on either side or on both sides by a thin plate of aluminium or other light metal or alloy. 7 The invention also consists in an electro acoustic device as set forth above wherein the main resonance of the device is controlled by electromagnetic damping.

The invention also consists in an electroacoustic device as set forth above wherein a coil is adapted to move in a magnetic field.

Further features of the invention will appear from the following description of a modification thereof which will be more readily understood by reference to the accompanying drawing representing a longitudinal section through the axis of a microphone constructed in accordance with the invention.

The principles upon which the invention is based will be set forth by consideration of a microphone consisting of a piston-like diaphragm caused to move by sound waves impinging thereupon. In order that this diaphragm shall be free from resonances due to its own flexure it is desirable to make it rigid. Also it is desirable that the mass of the diaphragm be as low as possible in order that it shall respond amply to the high frequency sound waves incident on it. One object of the present invention is to make a diaphragm which will meet the conflicting requirements of great rigidity and low mass, and this object is met if the diaphragm is made of a three-ply construction, consisting of two thin layers of a light metal such as aluminium enclosing a centre of light wood such as balsa wood.

If air is permitted to pass freely from the front to the back of such a diaphragm, the response of the device at all low frequencies will be impaired due to the diaphragm receiving almost equal variations of sound pressure on both its front and back faces. Therefore it is necessary to provide a closed cavity (or baffle) containing (or shielding) the air behind the diaphragm and to supply an air seal between the sides of the diaphragm and the sides of the air cavity. Such a seal for a microphone cannot in practice take the form of a frictional packing on account of the delicate nature of the movements of the diaphragm; and therefore in accordance with the invention a thin elastic surround is provided between the diaphragm and the sides of the cavity. The diaphragm then forms a rigid piston closing the mouth of a closed cavity, the edge of the diaphragm being sealed by a thin surround which stops the direct access of air to the cavity, but nevertheless permits the diaphragm to move. The surround which forms the air seal may conveniently support the diaphragm.

According to one embodiment of the invention the diaphragm has attached to its rear surface (i. e. the surface within the cavity and not exposed directly to incident sound waves) a coil which can move freely in a magnetic field provided by an electromagnet, or permanent magnet provided. This coil serves to convert the mechanical movement of the diaphragm into electric impulses and may also be utilized to provide damping for the resonance of the mass of the diaphragm and its elastic constraints. Such damping may be provided by closing the moving coil with a circuit having a low impedance at frequencies close to the resonance of the'oscillating mechanical system by which means an E. M. F. generated by the moving coil will cause damping currents to flow through the moving coil. It is thus possible to control the main resonance of the microphone and make the response such that it may be easily equalized by suitable electrical circuits. In order that this resonance may be well controlled, it is convenient to make the main resonant frequency as low as possible, and in order to do this it is necessary to make the surround thin and flexible. Such a thin flexible surround is liable itself to resonate due to high frequency sound waves impinging on it;

the resonance being due to the mass of the surround (and coupled air mass) resonating with the stiffness of the surround (and coupled air stiffnesslf .At the'irequency of this resonance the mass'reactance of the piston diaphragm is so high that it acts almost as a rigid support to the inner edge of the surround; but never suificient to modify the movement-of the piston diaphragm and so modify the response o'f the microphone.

the'response of the microphone is controlled and regular at high frequencies, and in order to dothis the surround may be shielded from direct access of high frequency Waves by means of a shield positioned close to the surround in front of it. Further the surround may he provided iii. DOi1lion;..-j Byyaryingtheamount of stretch a.

with damping in order to damp its" resonance, sinceii: its: resonance is well damped it will be unable te affect appreciably the I movementof the, comparatively heavy. piston diaphragm. Suchdamping may take the form of a rigid membe iixed. to the body of the microphone and extend ng over the surface ofthe surround, and veryclose to it, thus providing viscosity. damping dueto thethin film of air trapped between the r ou dsend h d member;

Such-jdamping ,must. not be confused with damping appliedto control the main resonance of the microphone. Thedegree (if-damping necessary to. control the resonance of the thin light surround ismuch smaller than that necessary.

' 5 i0 QOflfirolthemain resonance of the microphone.

Further, the-,;.value for the damping applied to the surround hasno very critical effect upon theresponse or the microphone, provided that thisdamping is suiiicient to reduce materially the effects of. the, surround resonanceon the movement of the piston diaphragm. Were an attempt made to control the main resonance by means of; this damping, the response of the microphone at its; main resonance would depend entirely .on

the dampingprovided; and also, With i116 degree o f viscqsitydamping easily obtainable, theresonancepeahwould be verysharpvand difficult to equalize. j

fience according to the inventionaf pistonlike l diaphragm is used, sealed. in themouth of a cavity a: thinflexible surround, which surround may also 7 serve to support the diaphragm. The; main resonance of themass-of the' piston diaphragn l and anything attachedtoit such as a moving coillwith its elastic constraint is controlledsby special-.-damping such aselectromagneticdamping, The resonanceor resonances of the thin surround; are prevented and/or con trolled byspecial shields interposed to prevent V the -incidence ofdirectqlif. F. waves'on thesur round andby special damping applied specifically to the ,surrpund. Such shielding may take the iorm ot a .membeiwextending over thefront of the surround and close to it. The-damping for.

the surround may take the form of a member 1 lying-very .close to either thel front' or back' of the surrpundand thus damping it due to theviscos'ity. of the trapped air; Alternatively members rnay. e-placed both in front: oiand behind itsoras todamp it more'efiectively-or a packing OfTQQttQD- WQOI ;or;-o ther damping. material may restlightlyagainst the surround. In order-that thethinsurroundmay be flatit isdesirable to arrange that it is suitably stretched whenin useful control of the main resonant frequency may be obtained.

Fig. 1 attached shows a section through the axis of'a microphone according to this invention constructed in accordance with the above principles. .Figure 2 represents in simple form a part of the circuit connected to the microphone theless a violent resonance of the surround is wherein the elements are arranged to provide electromagnetic damping for the diaphragm.

The diaphragm comprises a layer of balsa wood a: covered on each side by a thin sheet of aluminium, b, pressed and waxed together to form allowin a highly ri id light diaphragm to be constructed; or it, may be built up from a fiat diaphram or aluminium or other light alloy with cones of similar material attached to the back in .r order-to brace the diaphragm to rigidity. ihe

support may consist of a light elastic connection atfthe; edgesof the diaphragmandin the form shown. consists of an extended annulus c of the bacl :.alumin-ium covering sheet. This sheet. is pressed into the dish-shaped form shownin order that the plane of support by the annulus cf ma pass-through thezcentre of gravity of the system supported, For increased: flexibility the annulus 0 may be much thinner than the plate proper, this thinning being accom'plished, for example, after theplatesare fixed to the-wood, by dissolving off some oi; the metal with caustic soda. To the'back of the diaphragm may be attached a coild'iformedoi thin enamelledaluminium wire wound on a-thin aluminium former e which maybefixedto the .diaphragm byrivets f. In order to avoid short circuited turns the coil formen may havev in it a longitudinal saw cut extending almost to, the diaphragm'and' may, beforev use, be insulated by. a coating or" aluminiumhydroxide, i

. Convenientlythe microphonemay be more or less bottle-shaped (as shown). ,withia maximum diameter of, say, 3 inches, the body and neck of the bottle: forming the magneticsystem, while an attached holden'g, for-the; diaphragm and moving: coil, forms the bottom of the bottle. Thiseholder g may be fitted to the outer. pole piecejif Vofia small pot magnetby three, ormore,

centering screws 1'; and the diaphragm and mov;

ing coil secured against a step y" of. the holder, betweenvvashers k, by means of a screw thread-.

Held by screw :threads stretchingring a degreeyof control on the natural frequency of the diaphragm is also available. 7 As well as carrying the rounded ridge n, the stretching, member; forms a 'shieldover the thin surround, and to some extent serves to damp resonances-in; thesurround, It will be seen from the drawing'that the stretching ring has a por tion which. passes, very close to the surround-and iies clcse to the sloping; sides or the pistonportion. .This shieldserves, as explained, to reduce or prevent resonances in the surround. Increased dampingon the surround'can'be iobtained by making a small cavityb'etween'the stretching ridgenf and'the smooth portion ofthe shielding and damp n urface; such a; cavityi may; consist of a groove in the stretching ring immediately inside the stretching ridge n and such a groove may be say 1 mm. deep and 1 mm. broad. By making the damping surface lie very close indeed to the. surround, at a point just adjacent to this groove, it can be arranged that the damping isincreased due to small movements of the surround forcing air into the groove through this narrow neck between the damping surface and the surround. These effects may be enhanced by fitting another damping surface close to the surround at the back of the diaphragm.

To protect the diaphragm from accidental damage the aperture in the ring m through which the sound waves proceed may be formed only of anumber of small holes o drilled through the plate thus forming a grille which seems not to affect theacoustic properties of the device although serving admirablyfor protection.

The magnetic circuit as previously mentioned comprises a small pot magnet, and is formed of an outer pole piece h, the case of the microphone p',a rear plate, q, and a centre pole piece 1', on which are mounted the windings 5. Between the inner and outer poles is a gap (which is shown excessively large in the drawing forconvenience of illustration) in which the moving coil is positioned, and in order that the dimensions of this gap shall be correct a spacing ring t of brass or,

preferably, cadmium bronze, may be provided.

This may be a force fit on the inner pole piece and a good fit in a groove in the outer pole piece and serves not only to space the poles correctly but also to reduce the inductance of the moving coil, being magnetically coupled with it and having a low resistance.

The inner pole piece is preferably made of cobalt iron or other material having a high saturation density; but the flux density in the other portions of the circuit may be kept quite low, and good magnetic iron of thickness sufficient for the strength required may conveniently be used. The windings s fill the body of the microphone between the pole pieces, and are preferably formed of two difierent gauges of wire, the inner turns being of thinner gauge than the outer.

In order to avoid undue elastic restraint on the diaphragm by the air between it and the poles of the magnet it is desirable that a reasonably large volume be enclosed. To this end a cavity u .w' to damp resonance between the mass of air in the passage and the stiffness of the air in the main cavity. To avoid restraint due to air trapped within the moving coil former between the diaphragm and the inner pole piece holes may be drilled in the former as shown. To prevent short circuited turns in the magnetic held the coil former may have a saw-cut .r extending longitudinally over substantially its whole length. The leads from the moving coil as shown pass through the annular passage and cavity to terminals on the holder 9 but if desired the terminals may be located at any other suitable position such for example as at the rear end of the microphone.

The moving coil may be shunted by a circuit which in its simplest form consists of an inductance resistance and condenser in series, which circuit is designed to have a low impedance at frequencies close to resonance but a high impedance at frequencies remote from resonance, so that damping is provided electromagnetically but the efficiency of .the'device is not impaired at those frequencies where it is least efficient. Similarly other circuits may be used for this purpose as shown in Fig. 2 or as described in U. S. A. patent application No. 518,524, filed Feb. 26, 1931.

' In Figure 2 of the accompanying drawing the damping circuitshown is connnected in parallel with the microphone which is indicated by a symbolic representation of a coil 01 corresponding to the speech coil identified by the same reference letter in Figure 1.

Should it be desired to form a loud-speaker in accordance with this invention, this may be done with a construction very similar to that shown for the microphone, except that a horn or baiile may be fitted at the mouth in front of the diaphragm. The shield on the surround then prevents the surround generating an excess of high frequency waves when it resonates.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is:

1. An electro-acoustic device having a vibratory system comprising a substantially rigid piston-like diaphragm connected at its outer edge to a flexible air-sealing surround, electromagnetic damping means acting directly on the diaphragm to damp resonant vibrations thereof, and viscous means arranged to damp vibrations of the surround.

2. An electro-acoustic device having a vibratory system comprising a substantially rigid piston-like diaphragm supported solely by a flexible air-sealing surround connected to the outer edge of said diaphragm, electromagnetic damping means acting directly on the diaphragm to damp resonant vibrations thereof, and viscous means arranged to damp vibrations of the surround.

3. An electro-acoustic device having a vibratory system comprising a substantially rigid piston-like diaphragm connected at its outer edge to a flexible air-sealing surround, electromagnetic damping means acting directly on said diaphragm to damp resonant vibrations thereof, and a member positioned very near to said surround to trap a thin film of air between itself and the surround, said air film providing viscous damping on resonant vibrations of said surround.

4. An electro-acoustic device having a vibratory system comprising a diaphragm formed of light wood having a thin metal facing on each side, a flexible air-sealing surround extending from the outer edge of said diaphragm, electromagnetic damping means acting directly on the diaphragm to damp resonant vibrations thereof, and viscous means arranged to damp vibrations of the surround.

5. An electro-acoustic device having a vibratory system comprising a diaphragm formed of balsa wood having a thin metal facing on each side, said metal being extended beyond the wooden base to form a flexible air-sealing surround for the diaphragm, electromagnetic damping means acting directly on the diaphragm to damp resonant vibrations thereof, and viscous means arranged to damp vibrations of the surround.

6. An electro-acoustic device having a vibratory system comprising a substantially rigid piston-like diaphragm connected at its outer edge to a flexible air-sealing surround clamped at its perimeter to a rigid holder so that the diaphragm and surround cover the mouth of a closed cavity of large size, electromagnetic damping to damp vibrations :of the surround, said closed cavity comprising a portion immediately behind the diaphragm from which a narrow annular passage communicates with another portion more remotelyi disposed from the diaphragmwhich forms the main expansion chamber for the enclosed'air. I

8. An electro-acoustic device as claimed in claim 7' wherein, a packing of loose cotton .WOOl is inserted in said narrow annular passage to damp resonant oscillatorymovements of ,air between the. said two portions of the enclosed space. a j

9. An '-electro.-acoustic' device according to claim 1 comprising means for adjusting the tension of said surround 10. An -electro-acoustic device according to claim 1 comprising an adjustable stretching ring having a smooth ridge extending round said diaphragm adapted to press against said sur round for adjusting. thetension thereof 11. An 'electro-acoustic device according to claim 3 wherein said member adapted to damp said surround comprises an annular ring separated from the surround over at least one third of a its area by a distance not greater than one tenth viscous means to. dampjvibrations of said sur tenthof a; millimeter, said annular ring having agroove" cut in its surface adjacent :to the outer edge of said surround.

'13. An electro-acoustic device comprising substantially rigid piston-likev diaphragm, a former'attached to said diaphragm, anelectric coil wound on said former, means for providing a steadyamagrietic field in which saidvcoil is immersed, a flexible air-sealing surround eXtend-. ing from the edge of said diaphragm. and forming the sole means of support for said diaphragm assembly, electromagnetic means acting through the-mediumof said coil directly upon said diaphragm to damp resonant vibrations thereof, and

round.

14. An electroacousticdevice. according to claiml3 wherein. the centre of gravity of the Whole vibratory system comprising said diaphragm, former and coil lies in the plane of support of said surround; V

15. An ele'ctroc-ac oustic device according to claim 13 wherein said means for providing a steady magnetic'fieldcomprise a central inner magnetic pole piece projecting into said former, and an outer encircling magnetic pole piece surrounding said former, thus providing a field radially perpendicular to said former;

16. An electro-acoustic device having a vibratory system comprising a substantially rigid piston-like diaphragm connected at its; outer edge to aflexible' air-sealing surroundand an electric coil attached to said diaphragm, electromagnetic damping means actingdirectly upon said diaphragm and comprisinga' shunt circuit connected across said coil having an impedance which is low at frequencies in the neighbourhoodof mechanical resonanceof said vibratory system but high at frequencies removed therefrom, and viscous means arranged to damp vibrations of said surround. 7

"ALAN DOWER' BLUMLEIN.

HERBERTEEDWAED HO MAN. 

